Tire non-uniformity relates to the symmetry (or lack of symmetry) relative to the tire's axis of rotation in certain quantifiable characteristics of a tire. Conventional tire building methods unfortunately have many opportunities for producing non-uniformities in tires. During rotation of the tires, non-uniformities present in the tire structure produce periodically-varying forces at the wheel axis. Tire non-uniformities are important when these force variations are transmitted as noticeable vibrations to the vehicle and vehicle occupants. These forces are transmitted through the suspension of the vehicle and may be felt in the seats and steering wheel of the vehicle or transmitted as noise in the passenger compartment. The amount of vibration transmitted to the vehicle occupants has been categorized as the “ride comfort” or “comfort” of the tires.
Tire uniformity parameters, or attributes, are generally categorized as dimensional or geometric variations (radial run out and lateral run out), mass variance, and rolling force variations (radial force variation, lateral force variation and tangential force variation, sometimes also called longitudinal or fore and aft force variation). Uniformity measurement machines often calculate the above and other uniformity characteristics by measuring force at a number of points around a tire as the tire is rotated about its axis to generate a uniformity waveform.
Many different factors can contribute to the presence of uniformity characteristics in tires. Uniformity dispersions in tires can result from both tire harmonic uniformity effects and process harmonic uniformity effects. Tire harmonic uniformity effects have periods of variation that coincide with the tire circumference (e.g. fit an integer number of times within the tire circumference). Tire harmonic uniformity effects can be attributable to tread joint width, out-of-roundness of the building drums, curing press effects, and other effects. Process harmonic uniformity effects have periods of variation that do not coincide with the tire circumference. Process harmonic effects are generally related to process elements rather than tire circumference. Typical process harmonic effects can be caused, for instance, in the preparation of a semi-finished product (e.g. a tread band), by thickness variations due to the extruder control system or by rollers that can deform the shape of softer products. The impact of the process harmonic effect can change from tire to tire depending on the rate of introduction of the process harmonic effect relative to the tire circumference.
Tire uniformity parameters are typically measured or sampled at a plurality of equally spaced data points (e.g. 128 data points or 256 data points) for a rotation of the tire. The sampling of uniformity measurements at equally spaced data points can be used to construct uniformity waveforms for the tire. The uniformity waveforms can then be analyzed (e.g. by using Fourier decomposition or by estimating coefficients of a mathematical model modeling the uniformity measurement) to identify various uniformity attributes of interest, such as various tire harmonics and process harmonics of the uniformity parameter.
A problem with sampling the uniformity parameter of a tire at equally spaced data points about the tire can be that certain uniformity effects may not have maximum impact on the tire at the location of one of the discrete data points. This can cause errors in estimating the impact of certain uniformity effects contributing the uniformity of the tire, leading to reduced uniformity yield due to the fact more tires may exceed sorting limits.
As one example, a process harmonic may not manifest itself at a discrete data point when the uniformity of the tire is sampled at equally spaced data points around the tire. For instance, the peak of the process harmonic uniformity effect can be located between two sampled data points obtained for a tire. The peak of the process harmonic uniformity effect, however, will appear to be located at the nearest neighboring data point. Accordingly, an equally spaced sampling pattern may not naturally sample the process effect across a set of tires, leading to errors in estimating the process harmonic from uniformity measurements obtained using the equally spaced sampling pattern.
Thus, a need exists for improving the sampling of uniformity measurements about a tire to improve the estimation of uniformity effects, such as process harmonic uniformity effects.